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Ways of Expressing Population Growth
• Net birth rate = Births per unit time • Net death rate = Death per unit time• Net population increase = Net Birth rate – Net
Death rate• World Population mid-year 2007• 6.625 billion people
– Net birth rate/ year = 139, 125,000
– Net death rate/year = 59,625,000
– Net Increase/year = 78,500,000– USA Population mid-year 2007 = 302,200,000
• World Increase = USA Population - 3.85 years
Expressing Population Growth
• Births/1,000 =
• (Births per year/ mid-year population size) X 1,000
• Deaths/1,000 =
• (Death per year /mid-year population size) X 1,000
• Natural Rate of Increase =
• Births/,1000 – Death/1,000
World Population mid-year 2007
• Birth rate = 21
• Death rate = 9
• Rate of Natural Increase = 12
• Percent Growth = – 1. Rate of Natural Increase/10– 2. 12/10 = 1.2%
• Approximate doubling time =
• 70 years/ % Growth
• World Doubling Time =70 yrs/1.2 % = 58 yrs
Comparison of Ways to Express Population Growth
A. Population size = 1,000 B. Population size = 1,000,000
1. Net birth = 10 Net births = 100
2. Net deaths = 5 Net deaths = 50
3. Net increase = 5 Net increase = 50
1. Birth /1000 = 10 = 0.10
2. Death/1000 = 5 = 0.05
3. Rate of natural increase = 5 = 0.05
4. % growth = 0.5% = 0.005%
Rate of Natural Increase and % Growth do not include net immigration
• United States
• Birth per 1000 = 14
• Deaths per 1000 = 8
• Net Immigration per 1000= 4
• % Growth with Immigration = 18-8 = 1.0%
• % Growth w/o immigration = 14-8 = 0.6%
Population Growth(Basic Growth Curves)
• “J-shaped” growth or exponential growth
• Sigmoid or “S-shaped” growth
EXPONENTIAL GROWTH
• Population increase – Fixed percentage of population size at the
beginning of the period
• Example of exponential growth is geometric growth - – Growth by doubling – Fixed percentage is 200 percent
Geometric Growth of a Sheet of Paper• Number of doubles Thickness
• 1 2/254 inch
• 2 4/254
• 3 8/254
• 4 16/254
• 5 32/254
• 6 64/254
• 7 128/254
• 8 256/254
Geometric Growth of a Sheet of Paper
• Number of doubles Thickness
• 12 1.25 foot
• 20 340 feet
• 35 3,000 miles
• 42 384,000 miles (reaches the moon)
• 50 93,000,000 miles (reaches the sun)
Exponential Growth
• Initially, no limit placed on growth• Population grows at max possible rate
– Intrinsic growth rate (biotic potential) • r = biotic potential• Maximum rate/individual
• Growth is halted abruptly:– Depleted resource (food or space)– Epidemic disease
• Environmental resistance – Applied all at once
J-shaped Growth Curve
Environmental
resistance – applied
abruptly
Time
Carrying Capacity
PO
PU
LA
TIO
N S
IZE
J-shaped growth curves are characteristic of populations in environments of
• Low diversity
• Few predators or competitors (few ecological controls)
• Simple food webs
Sigmoid Curve
• Detrimental factors (environmental resistance) – Increases in proportion to population
increase
• Characteristic of environment– High diversity – Predators, competitors exert biological
control
AGE STRUCTURE DIAGRAMS
• Graphically – % of individuals within various age categories
• Three age categories
• Prereproductive (Age 0-14 years)
• Reproductive (Age 15-44 years)
• Postreproductive ( Age 45+)
• Plot males on left and females on right
Age Structure DiagramsAge
% males in theage group
% females in theage group
Post-reproductive
Reproductive
Pre-reproductive You
nger
to o
lder
What Does the Age Structure Diagram Indicate?
• Growth Patterns
• Proportional Distribution in Age Categories
Pre-reproductive
Reproductive
Post-reproductive
Expanding Growth 2.7%
Stabilizing 0.6%
Diminishing –0.2%
Changes in USA Age Structure
• Last century USA – Expanding population to a stabilizing population– Total fertility rate (2007 USA 2.1)
• Post war baby boom (1946 and 1964) – Peaked 1955-1959– 75 million bulge in population – Large affect on social and economic structure
Effect of Post War Baby Boom
• 1960-1975 – expansion of schools
• Late 1970’s – 1980’s high unemployment
• 2,005-2025 – Dominance of middle age (Pension cost begin to rise)
• 2,025 – 2,040 – Period of Senior Citizens
Changes -USA Birth Rates and Death Rates
• Year BR DR RNI %Growth • 1947 26.6 15.0 11.6 1.16• 1977 14.7 9.0 5.7 0.57• 1987 16.0 9.0 7.0 0.70• 1996 14.6 8.8 5.8 0.58• 2000 15.0 9.0 6.0 0.60• 2004 14.0 8.0 6.0 0.60• 2007 14.0 8.0 6.0 0.60• Over the last 50 years births and death rates have declined • BR = Birth Rate, DR = Rate, RNI = Rate of Natural Increase
Implications of Death Rates and Birth Declining
• Fewer Births = fewer young people
• Fewer Deaths = More Older People
• Birth rates declined more rapidly than death rates = fewer young people more older people
Results of declining growth in USA • USA has an aging population:
– Proportionately fewer young people and more older people
• Median Age of the USA Population
• 1970 -- 29 years
• 1990 -- 33
• 2,000 -- 36
• 2010 -- 39
United States’ Aging Population
• Population 1950 1985 2020• Ages 65-84 (Millions) 11.7 25.8 44.3• 85 and over (Millions) 0.6 2.7 7.1• 65 and older % total 7.7% 12% 17.3%
• LIFE EXPECTANCY
• Total Pop. (years) 68.2 74.7 78.1• Federal Spending• Pension & Health-care
As a % of GNP 1.6% 9.3% 11.8%
Social Security
• Continued adjustments in Social Security Social Security – Not a pay as you go system.
– Pay while you work get benefits later
• As the populations ages:– Future retirees will have few workers
supporting their retirement than current retirees or retirees in the past
Ratio: retirees/worker
• Date Retiree/workers
• 1950 1/16
• 1960 1/5
• 1965 1/4
• 1985 1/3
• 2025 1/2
Social Security: Current Status
• 2018 – More expenditures than income
• 2042 –Trust Fund Depleted– Pay out all in coming funds– 75 % of current benefits
• Reality Trust fund is not fully funded
Suggested Solutions
• Raise FICA Tax (Federal Insurance Contributions Act) – Current (7.65%)
• Tax income over $90,000 (current cap)
• Tax one-half of Social Security Income over $32,000
• What changes have occurred?
Birth Year & Age for Full SS Retirement Benefits
Birth Year Age for full benefits
1937 or earlier 65
1939 65 and 4 months
1941 65 and 8 months
1943-1954 66
1955 66 and 2 months
1957 66 and 6 months
1959 66 and 10 months
1960 and later 67
Population Structures by Age and Sex, 2005 Millions
300 100 100 300300 200 100 0 100 200 300
Less Developed Regions
More Developed Regions
Male Female Male Female
80+ 75-79 70-74 65-69 60-64 55-59 50-54 45-49 40-44 35-39 30-34 25-29 20-24 17-19 10-16
5-90-4
Age
Source: United Nations, World Population Prospects: The 2002 Revision (medium scenario), 2003.
Age Distribution of the World’s Population
Provided online by the Population Reference Bureau Graphics Bank, The Graphics Bank was prepared by Allison Tarmann, senior editor, and Theresa Kilcourse,senior graphics designer. Please visit www.prb.org.